Dual propeller drive for ski boat

ABSTRACT

A dual prop drive for a ski boat has a gear box having an input gear that is connected to an engine output shaft. The gear box has two laterally spaced output gears on opposite sides of the input gear, with the output gears being drivingly connected to the input gear. The dual prop dive unit has two propeller shafts, each of which is drivingly connected to one of the output gears. The output gears and propeller shafts desirably rotate in counter-rotating directions. The gear box desirably reduces the rate of rotation of the propeller shafts relative to the engine output shaft. The gear box is lubricated without a pump by a gravity feed splash oil system.

CROSS-REFERENCES TO RELATED APPLICATIONS

This is a non-provisional application based on and claiming the filingpriority of co-pending provisional patent application Ser. No.60/489,746, filed Jul. 23, 2003.

BACKGROUND OF THE INVENTION

The present invention relates to an improved drive mechanism for arecreational motor boat having a planing hull, such as the type used forwater skiing and the like, and a method for retrofitting an existingmotor boat to incorporate the improved drive mechanism.

As used herein, reference to a “recreational motor boat” is intended toinclude the class of boats having planing hulls, which are larger thanrow boats and small boats with outboard motors. Generally such boats aresmaller, than larger, non-planing cabin cruisers, yachts, and largeships. These boats are typically in the range of 16-28 feet in lengthand frequently are powered by a single inboard engine that turns apropeller by means of a rotating propeller shaft that extends throughthe bottom of the boat. The propeller shaft is journalled in bearings ina support bracket mounted on the underside of the boat.

Boats with planing hulls used for water skiing or other water sportswherein one or more persons are pulled behind the boat (all referred toherein as water skiing) require a substantial amount of low speed powerto pull water skiers out of the water and a substantial amount oftop-end speed in order to pull the skiers at a high rate of speed wherethe boat hull is planing. As an example, one well-know water ski boatthat employs a 350 horsepower automobile engine is currently able topull up to fourteen skiers or more out of the water and then reach a topspeed of 42 to 51 mph at a maximum rated engine speed of 5100 rpms. Topspeed for most skiing demonstrations, however, is somewhat less. Boatsused for related water sports, such as wake boarding has comparable orgreater performance requirements. Larger boats, such as 28-foot boats,are heavy and require greater power than smaller boats.

At the present time, the best professional water ski boats employ asingle inboard engine that drives a single prop. In order to increasethe performance capabilities of such a boat, it is necessary to increasethe engine size substantially in order to achieve both increased powerand increased top speed. This increases dramatically the cost of the skiboat. Moreover, even with a more powerful engine, a boat with a singleprop can still pull no more than about fourteen skiers.

Twin props have been used on ships and on various boats for specificpurposes, but they require greater power. Increased power requires alower engine to prop gear ratio. This in turn was thought to produceunacceptably reduced top speeds, at least without the added expense ofthe two-speed transmission. Two prop drives also have required aredesign of the boat interior and have not been adaptable as a retrofitin an existing one prop boat. An object of the present invention is toprovide an improved and cost effective drive train for a recreationalmotor boat of the type used in water skiing in order to improve theperformance characteristics of the boat.

BRIEF SUMMARY OF THE INVENTION

In accordance with the present invention, an improvement in the drivetrain for a water ski boat employing a single engine comprises fittingthe boat with a gear box wherein the single drive shaft on the inboardengine drives at least two laterally-spaced props at a reduced gearratio. An additional feature of the present invention is that improvedsteering and handling characteristics are achieved by interconnectingthe two propellers so that the propellers are counter-rotating (rotatein opposite directions).

These and other features and advantages of the present invention willhere and after appear and for purposes of illustration but not oflimitation a preferred embodiment of the present invention is describedbelow and shown in the appended drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic side elevational view of a motor boat employingthe dual prop drive of the present invention.

FIG. 2 is a partially broken away plan view of the boat of FIG. 1,showing in phantom the construction of a single prop drive not employingthe present invention.

FIG. 3 is a plan view of the gear box of the present invention.

FIG. 4 is a front elevational view of the gear box of FIG. 3.

FIG. 5 is a sectional view taken along line 5-5 of FIG. 3.

FIG. 6 is a sectional view taken along line 6-6 of FIG. 3.

FIG. 7 is a sectional view taken along line 7-7 of FIG. 3.

FIG. 8 is a plan view of the Gearbox of the present invention.

FIG. 9 is a perspective view of the gearbox with the covers removed.

FIG. 10 is a perspective view of an output gear shown in the gearbox,with a funnel-type gravity feed lubricator being positioned to receivelubrication splashed in the gear housing by the rotating gears, thelubricator conveying lubrication by gravity to the idler and input gearsin the interior of the housing, the zerk fitting on the thrust bearingalso being removed so the thrust bearing can receive splashedlubrication instead of requiring manual lubrication.

FIG. 11 is a perspective view showing the side of the gearbox housingfacing the engine, showing a removable access door on the housing foraccess to the ends of the propeller shafts to fasten them into thehousing.

FIGS. 12-14 are perspective views of the underside of the boat hull,showing the propeller shafts, propellers, and rudders, and showing thelocations and fittings for the original single prop propeller shaft.

DETAILED DESCRIPTON OF PREFERRED EMBODIMENTS

A motor boat or powerboat 10, shown schematically, includes an inboardengine 12, such as a 350 horsepower automobile engine or larger, mountedin a conventional manner inside the hull 14 of the boat. The engine hasa conventional transmission 16, which typically permits the engine to beshifted between neutral, forward, and reverse gears. In the exemplaryembodiment the engine transmission includes a gear reduction ratio of 1½to 1, which means that when the engine is rotating at 3,000 rpm, theengine output shaft is rotating at 2,000 rpms.

A conventional inboard engine (which for convenience will be deemed toinclude the conventional engine transmission) drives the boat by meansof a propeller shaft 18, which extends downwardly and rearwardly fromdriving engagement with an output shaft 17 of the engine through thebottom of the boat to an outer end, on which a propeller is mounted. Thepropeller shaft is journalled in seal 19 in the floor of the boat and ina support bracket 20 mounted on the underside of the boat. A rudder 21is positioned behind the propeller. This is the setup for a conventionalrecreational boat.

In order to achieve additional power without redesigning the whole boatand without sacrificing significant top end speed, the propeller shaft18 and propeller 22 of a conventional system are replaced by a dual propdrive mechanism constructed in accordance with the present invention.Dual prop drive mechanism 24 comprises a gear box 26 driven by theengine and a pair of laterally-spaced propellers 28 and 30 driven bypropeller shafts 32 and 34, which are in turn driven by output gears 36and 38 in the gear box. The output gears are driven by an input gear 40that is drivingly connected to the output shaft or drive shaft of theengine through transmission input shaft 25. Input gear 40 is connectedto output gears 36 and 38 by idler gears 42, 44, and 46. There is asingle idler gear between input gear 40 and output gear 36, and thereare two idler gears connected together between input gear 40 and outputgear 38. The extra idler gear between input gear 40 and output gear 38causes output gear 38 to rotate in the opposite direction from outputgear 36. Thus, propellers 28 and 30 rotate in opposite directions.

A separate rudder 27 and 29 is placed behind each propeller 28 and 30for steering the boat. The main rudder 21 can be disconnected (and leftin place if desired) or it can be left in operational condition.

The counter rotating props provide important steering and handlingadvantages. With a single prop or with a dual prop setup wherein bothprops rotate in the same direction, it is easier to steer in onedirection than the other direction. In addition, steering in reverse iseven more difficult. Typically, without counter-rotating props, a boatcan be steered in reverse in only one direction. With dual,counter-rotating props, the boat can be steered equally easily in eitherdirection, whether the boat is in forward or reverse gears.

The gears in gear box 26 are housed in a housing 48, which has anon-removable top over the center portion of the housing and removablecovers 50 and 54 on the sides, which provide access to the interior ofthe housing. The housing is fluid tight and is provided with an oil bathfor cooling and lubricating of the gears. A dip stick 51 mounted incover 50 is used to measure the oil level in the housing.

A feature of the invention is that lubrication is circulated over thegears in the housing without the use of a pump by means of a gravityfeed splash oil funnel 31 that receives oil being splashed by the gearson the sides of the housing and feeds the oil into the central interiorof the housing. This gravity feed lubrication system avoids the expenseand pump failure problems of a pump lubrication system.

The gear box is shaped so as to be large enough to hold the gears andyet be small enough that it fits in a conventional motor boat withoutsubstantial modification of the hull or repositioning of the engine. Inpractice, however, it may be necessary to modify slightly the bottom ofthe hull by putting a slight bulge 15 therein, in order to fit a gearbox into a conventional motor boat that is retrofitted to accommodate adual prop drive unit. As shown, the housing of the gear box preferablyis a durable metal, such as cast aluminum, and the covers are securelybolted to the gear housing. This prevents injury in the event that gearsshould fail while the engine is operating at a high rate of speed.

Propeller shafts 32 and 34 extend inwardly through an outer wall 56 ofthe gear box and then through gears 36 and 38. The propeller shafts arethen received in thrust bearings 58 and 60. Thrust bearings 58 and 60receive the forward thrust exerted by the propellers on the propellershafts and translate the forward thrust into forward movement of theboat. The zerk lubrication fittings conventionally in thrust bearingsdesirably are removed, leaving openings 59 that receive lubricant bygravity as the oil is splashed around the interior of the housing by thegears.

While a gear box is desired in the present invention, because ofdurability and high speed operation attributes, the same functionalcharacteristics can be achieved with a roller chain and sprocketarrangement, although durability is more of a concern with a rollerchain and sprocket arrangement operating at the high engine speedsemployed with a gasoline engine. Also, it is possible to drive twinpropeller shafts by means of a hydraulic drive mechanism, wherein theboat engine drives a hydraulic pump, and the pump drives individualhydraulic drive motors for each of the propeller shafts. Whichever driveis employed, the desired drive ratio still should be employed.

As shown in the drawings, if the present invention is retrofitted in anexisting installation, it is not necessary to completely remove thecenter propeller shaft 18 from the engine compartment, although this ispreferred if the center shaft is not used. Instead, the propeller shaft,which is sealed in a propeller shaft opening in the hull of the boat,can simply be disconnected from the engine and possibly shortened sothat gear box 26 will fit in the hull behind the engine. The propellershaft in its existing position thus makes it unnecessary to plug a newopening in the bottom of the boat. The propeller can be removed from theend of the propeller shaft in order to reduce drag. Obviously thepropeller shaft can be removed in its entirety and the hole in the boatwhere the propeller shaft extended through the hull can be sealed. Thesupport bracket for the main propeller shaft can be left on bottom onthe bottom of the boat or can be removed as desired. As a possiblealternative, the main propeller shaft can be connected to the input gearof the transmission, so that the boat operates with three propellers.

The gear ratios chosen for the present invention are significant featureof the invention. Desirably, the gear ratios of the gears employed inthe gear box are selected so that the rotational speed of the enginetransmission output shaft and the dual propeller shafts are about 1½ to1, which is when the output shaft from the transmission is rotating at3,000 rpms, the dual propeller shafts are each operating at 2,000 rpms.

With a 1½ to 1 gear ratio in the gear box and the gear box beingconnected to an engine having a conventional 1½ to 1 gearing in theengine transmission, the overall gear reduction from the engine to theprop is 2¼ to 1. This exact ratio can be varied, as long as the range isset to provide sufficient power to pull the desired number of skierswithout losing so much top boat speed that desired boat speed or waterskiing performances cannot be achieved. A range of operable gear boxgear ratios is about 1.2 to 1.8 to 1, yielding an overall engine to propratio of about 1.8 to 2.7 to 1.

While conventional wisdom would suggest that the reduction in gearratios employed in the present invention would cause the boat to be tooslow, it has been surprisingly found that the overall speed of the boatis not substantially reduced by this reduction in gear ratios. On theother hand, the power of the boat is increased dramatically.

The propellers in the exemplary embodiment of the present invention havea diameter of 14 inches and a pitch of about 16 to 18. This is thepreferred propeller size, but the exact size has not been determined tobe critical. The pitch of the prop is related to speed, so a change inpitch can have the same effect as a change in gear ratio. With the 1½ to1 gear box ratio, a pitch of 18 may produce a top speed of about 43 mph,while a pitch of 16 may produce a top speed of 38 mph. A range ofpropeller sizes about 10 to about 18 inches in diameter is possible. Alimitation in propeller size is that the propellers must not contacteach other or the hull of the boat. A propeller size of about 10 inchesin diameter is desired for a three-prop installation, so the props donot overlap.

With a dual prop system employing two 14 inch props having a pitch of16-18 and employing a gear box ratio of 1½ to 1 (2.25 to 1 overall fromengine to props), it has been found that the performance increase isdramatic. Whereas a conventional single prop water ski boat can pull amaximum of about 14 skiers from the water, no matter how powerful theboat, a boat retrofitted with (or fitted in the first instance with) thegear mechanism of the present invention can pull 22 skiers from thewater without any problem.

Moreover, the top speed is not substantially reduced. Whereas a singleprop boat may achieve a top speed of approximately 42 to 51 mph at amaximum rated engine speed of 5,100 rpms, the same boat fitted with thedual prop drive of the present invention can achieve a top speed of 38to 43 mph, which is generally well above the speeds necessary for mostwater-towing sports. With the improved drive mechanism of the presentinvention top boat speed is even high enough to pull barefoot skierswhich usually require the highest boat speed.

While a 1½ to 1 gear box ratio is desired in the present invention andhas been demonstrated to be effective, variations in the gear ratios maybe employed, as desired, in order to alter the performancecharacteristics one way or another to achieve greater power or greatertop-end speed. In order to achieve the 1½ to 1 gear ratio in the presentinvention, the drive mechanism can include an input gear 40 having adiameter of 4 inches and output gears 36 and 38 having a diameter of 6inches, with the idler gears 42, 44, and 46 having diameters of 5inches.

One of the advantages of the improved drive mechanism of the presentinvention is that the drive mechanism does not appear to affect the wakeof the boat. In a water ski boat, it is usually desirable to minimizethe wake of the boat (unless the boat is used for wake boarding or otheractivities). When a dual prop mechanism is employed, the wake issubstantially the same. In order to achieve the additional power byusing two engines, substantial additional weight would be added as wellas expense, and the additional weight would increase the wake of theboat.

Another important advantage of the present invention is that the drivemechanism provides increased pulling power with little decrease in speedunder load. Thus where one or a large number of skiers are pulled, themaximum speed of the boat is not reduced nearly as much as when a singleprop engine is used. This increased pulling power also makes it easierto maintain a constant speed through a slalom course, even though thisload varies substantially.

Likewise, when a skier is pulled from a sitting position on a dock, thetow boat does not lose momentum due to prop slip when the rope getstight. A strong steady increase in speed is achieved, and this minimizesfalls.

Another outstanding advantage of the two propeller drive is when a barefooted skier steps off his ski at 40 mph, the boat speed is notdramatically reduced like it is with a single prop.

The present invention also is advantageous for wake boarding. A wakeboard is different from a water skiing operation, because a larger wakeis desired. In order to achieve a larger wake with a conventional skiboat is customary to add ballast to the boat, which causes the boat toride lower in the water and thus increases the wake. More power isnecessary to drive a boat when it is riding lower in the water, and thedrive mechanism of the present invention provides the additional power.

While the present invention is particularly desirable with a water skiboat, the advantages of the present invention can also be achieved withlarger family boats as long as about 28 feet or so in length which aregenerally used only recreationally for water skiing. Boats of thisnature sometimes are driven by a single prop and a single motor, butthese boats can be underpowered. An option for a larger boat is the useof two engines, but this is considerably more expensive and adds weightto the boat. Good performance characteristics, including good power andgood top speed, can be achieved with a single engine in a boat as largeas 28 feet or larger by employing the dual-prop drive mechanism of thepresent invention.

In the foregoing exemplary embodiment the transmission is mounted to therear of the engine and in front of the props. The present invention alsocan be used in installations where the engine is mounted to the rear ofthe transmission, in which case the engine output shaft faces forwardlyand is connected to the rear side of the transmission. The props alsoare connected to the rear side of the transmission.

It will be understood by one having ordinary skill in the art and bythose who practice the invention that various modifications andimprovements may be made without departing from the spirit of thedisclosed concept. Various relational terms, including left, right,front, back, top, and bottom, for example, is used in the detaileddescription of the invention and in the claims only to convey relativepositioning of various elements of the claimed invention. The scope ofprotection afforded is to be determined by the claims and by the breadthof interpretation allowed by law.

1. A method for enhancing the pulling power of a single enginerecreational motor boat while maintaining desirable top speedperformance, wherein the boat comprises an inboard engine that rotates apropeller through a propeller shaft, the method comprising the steps of:disconnecting the main propeller shaft from the engine; providing a gearbox having an input gear adapted to be drivingly connected to the engineoutput shaft and having two laterally spaced output gears on an oppositesides drivingly connected to the input gear, the gear box being fittedin a boat hull adjacent the output shaft of the engine, with the inputgear being mounted on an input shaft that is in driving engagement withthe engine output shaft; and drivingly connecting output gears tolaterally spaced propellers positioned on the exterior of a boat, suchthat the propellers are driven by the output gears through propellershafts that extend through laterally spaced openings formed in thebottom of the boat.
 2. A method as in claim 1 wherein a gear box outputshafts, on which the output gears are mounted, are drivingly connectedto the gear box input shaft in such a way that the gear box causes theoutput shafts to rotate at substantially the same rotational speed aseach other in counter-rotating directions, the gear box reducing therate of the rotation of the output shafts relative to the rate ofrotation of the input shaft, the output shafts being connected topropeller shafts that extend through the bottom of the boat from innerends that are driven by the gear box to outer ends that are spacedlaterally under the boat;
 3. A recreational motor boat having a planinghull wherein the boat has a single engine and a single engine outputshaft, the improvement wherein the boat includes a gear box having aninput shaft connected to the engine output shaft, the gear box havingtwo spaced output shafts that are driven in counter-rotating directionsby the input shaft through gears in the gear box, the output shaftsbeing drivingly connected to spaced propellers mounted on the exteriorof the hull, the gear box reducing the rotational speed of the outputshafts in comparison with the input shaft.
 4. A boat as in claim 3wherein the gear box gear ratios reduce engine output shaft rotation bya factor of 1½ to 1.